The present invention relates to a device for sensing a motion of an object to be monitored, and particularly to a device for mechanically and electrically sensing a fluctuation such as inclination, swing, or the like, of an object to be monitored and detecting the fluctuation in the form of an electric signal.
For example, with respect to a parked car in which no one rides, there are possibilities of robbery of the car per se, robbery of a tire, rear-end collision by another car, mischief, etc. Conventionally, there has been provided a robbery-proof or anti-theft device attached on a car in its car body or trunk room for sensing a fluctuation such as inclination or swing of the car and for producing an alarm sound when the fluctuation is sensed. Such a robbery-proof device comprises an electrically conductive cantilever spring with one end thereof being fixed in a casing and with the other end thereof being attached with a weight, a contact attached at a middle portion of the cantilever spring, and a support member on which a fixed contact is attached at a position opposite to the first-mentioned contact with a predetermined distance therebetween. The car body is inclined or swung by a weight load of a trespasser into the car in case of robbery of the car per se, and by jacking up in case of robbery of a tire. The cantilever spring in the robbery-proof device is caused to vibrate by the inclination or swing of the car so that the contact attached to the spring intermittently comes in contact with the fixed contact. Upon contacting between the two contacts, an alarm device is operated to inform the owner of the car the robbery. In order to sense slight inclination or swing of the car body, however, it is necessary that the cantilever spring has sharp vibration resonance frequency characteristics. Therefore, there has been a defect that the contact between the two contacts can not be attained when an inclination angular speed, or the like, of the car is out of the frequency of natural vibration of the cantilever spring. Besides, the frequency of vibration of a car body varies depending not only on the kind of a car and the weight of load, but also on the weight of a transpasser. Therefore, it has been impossible to set the cantilever spring to operate in wide frequency range for all the kinds of inclination and swing.